Clarity On Tire Clearance
How does Cervélo determine tire clearance?
Recent advances in wide rim technology and the rising popularity of options beyond the formerly ubiquitous 23c tire have prompted Cervélo’s engineering team to update our definition of tire clearance, starting with the development of the S5.
Figure 1: View of the underside of the chainstays and bottom-bracket from the S5 (in red), with the tire (in dark blue) and tire clearance (in light blue).
The image above depicts our tire-clearance model for an S5 frameset. The frame, in red, is manufactured using a hard-tooled mold, which means the surfaces will not vary from the initial design. UCI rules permit the frameset shape to vary by 1mm from the shapes sent in for approval, which is functionally the allowance for variances in paint, bonding material, and other normal manufacturing variability. The dark-blue radius is the tire itself, which in this case corresponds to the width of a 25mm tire on a 23mm-wide rim. When the S5 was being designed these were the largest tires, on the largest internal-width rim, we felt were likely to be used. Once we identified the biggest tires riders might be expected to fit in the frame, we calculated the remaining clearance required to accommodate factors such as manufacturing tolerances, debris clearance, and the space needed to meet CPSC and ISO legal requirements. This is represented by the light blue area, surrounding the tire, and is referred to as the clearance area. The extent of this area can vary depending on a frame’s design and intended use. The S Series’ clearance area, for example, is slightly smaller than the R Series’ owing to the former’s aerodynamic requirements and seat-tube cutout.
Figure 2: Rear view of tire, mud guard, and chainstays on the C5.
Above, you can also see that clearances are different between different parts of the C5 frame. There is actually more space between the seatstays and the tire (Fig. 2) than between the chainstays and the tire (Fig. 1). This tells us that if a customer uses a tire larger than our recommended dimensions, they would first see rubbing on the chainstays.
When it came time to evaluate the tire clearance needs for the new C Series frames, it became clear that we needed to refine our design approach. There are many who believe that “bigger tires are better” in the world of endurance road riding, but this does not necessarily align with the need to ensure adequate bottom-bracket stiffness. In fact, these two requirements are often diametrically opposed, because they are competing with the chainstay for space in the limited area between the chain rings and the rear tire. Wider chainstays boost stiffness, but at the expense of tire clearance.
For the C Series we expanded on the concept of starting with a design-basis tire dimension by specifying a maximum tire-plus-clearance model when we initiated the design. In the past, we attempted to find the largest tire possible in a given size, install it on the largest rim, and establish that as our design goal. However, we found that there were simply too many variables in the mixed-surface category to continue with this method. Moving forward, instead of stating that “all such-and-such-size tires” will fit, we will provide the actual design-basis tire dimensions chosen for the bike model (in the form of wheel diameter and installed tire width), and strongly recommend that these dimensions are not exceeded. These dimensions will correspond to the dark blue radius in the drawings.
NOTE: When considering a prospective tire’s compatibility with the frame, it’s important to respect the maximum measured size of tires, rather than relying on the manufacturer's nominal values, which are dependant on real-world tire pressure, loading, rim width, rim ERD, and tire-mold accuracy.
Failing to adhere to these design-basis tire dimensions can result in damage to the frame or fork. Circumstances that promote wheel flex under hard efforts, mud or ice buildup during spring riding, or picking up a rock from a freshly paved road, may cause an oversized tire to impact the frame. These are all realistic risks and should be taken seriously. The purpose of the light blue clearance area is to account for these variables.
The addition of mudguards also confuses the tire clearance question, as they are not usually included in the requirements for clearance; again, there are too many variables to consider. In the case of the C Series, we used the SKS P35 as our design-basis mudguard.
How (Actual) Tire Width is Measured
- The tire is inflated to maximum recommended inflation pressure (as listed on the tire sidewall).
- Using a calibrated vernier caliper, the width of the tire at the widest point is measured at four points (12, 3, 6, and 9 o’clock). The resulting widths are averaged to give the final result.
How (Actual) Tire Diameter is Measured
We believe the industry best practice to measure diameter is to perform a tire rollout test.
- The tires are inflated to maximum recommended inflation pressure (as listed on the tire sidewall).
- To get an accurate result it is critical to keep the bike straight during the rollout. We built a metal track that ensures the bike stays straight, and is long enough to accommodate five wheel revolutions.
- The starting point is marked on the tire and the bike is rolled for five complete wheel revolutions.
- We repeat this rollout five times to ensure accuracy.
- The resulting distances for the five rollouts are averaged, and then are divided by five to give one wheel circumference. Circumference is translated into diameter by dividing by pi (𝛑) or 3.1416.
What does this mean? This means that C series frames are compatible with a 28mm tire with a mud guard or a 30mm without a mud guard (with sufficient room for debris).